Fix error message for non-defined objects in planner_objective

var n_bar S_bar;

model;
n_bar=1;
end;

planner_objective(chi*n_bar);

returns

ERROR: You cannot include exogenous variables in the planner objective. Please define an auxiliary endogenous variable like eps_aux=epsilon and use it instead of the varexo.

due to chi not being defined as any type.